1. Field of the Invention
The present invention relates to a power unit for a two-wheeled vehicle coupled in a freely swingable manner to a vehicle frame. In particular, the present invention is directed to improving a power unit provided on an engine with a crankshaft arranged in a lateral direction of the vehicle, a transmission case extending rearward from one end in a horizontal direction of the crankcase of the engine and axially supporting a rear wheel at a rear end part thereof, and a transmission housed in the transmission case coupling the crankshaft of the engine and the rear wheel.
2. Background Art
Conventionally, this kind of power unit for a two-wheeled vehicle is constructed for single cylinder engines; for example, refer to Japanese Patent Laid-open Publication No. Hei. 4-163287. The main object of the conventional art is to prevent the occurrence of interference of a power unit with a road surface during banking or rocking of the two-wheeled vehicle. This is accomplished by arranging an exhaust purifying device adjoined to an exhaust pipe of the two-wheeled vehicle which is positioned above a central line extending in a longitudinal direction as seen from a side view of the two-wheeled vehicle. Positioned thereunder is the power unit, being of the rockable swing type, such that the lateral placement thereof is sufficiently drawn inward to the body of the two-wheeled vehicle in order to prevent contact with the road surface when the two-wheeled vehicle is banked.
However, due to the positioning of the exhaust purifying device adjoined to the exhaust pipe, to allow placement of the power unit close to the frame of the two-wheeled vehicle, the conventional art two-wheeled vehicle is limited to single cylinder engines. Therefore, it is not possible to increase the engine power output by readily adopting a two cylinder engine configuration. Due to the lack of positioning freedom of the cylinders of the two cylinder engine caused by the exhaust purifying device adjoined to the exhaust pipe, the cylinders of the two cylinder engine can not be freely set without interfering with a crankcase of the two cylinder engine. Thus, implementation of a two cylinder engine with the two wheeled vehicle of the conventional art complicates a manufacturing cycle of the vehicle and further complicates assembly of the crankcase and cylinder block of the two cylinder engine.
In order the overcome the clear disadvantages of the conventional art, it is the object of the present invention to provide a power unit for a two-wheeled vehicle where the direction of the cylinders can be freely set without interfering with a crankcase of an engine so as to provide greater freedom of design when there is an intent to increase the engine power output by adopting a two cylinder configuration, and where manufacture and assembly of the crankcase and cylinder block are straightforward.
In order to achieve the above object, in a first aspect of the present invention, a power unit for a two-wheeled vehicle coupled in a freely swinging manner to a vehicle frame comprises an engine with a crankshaft arranged in a lateral direction of the vehicle, a transmission case extending rearwards from one end in a horizontal direction of the crankcase of the engine and axially supporting a rear wheel at a rear end part thereof, and a transmission housed in the transmission case coupling the crankshaft of the engine and the rear wheel. The crankcase is divided by a partitioning surface passing esentially through a central part of the crankshaft in the axial direction in such a manner that the crankcase comprises a first crankcase half and a second crankcase half. The transmission case is integrally formed with the first crankcase half and a cylinder block having two cylinders running parallel with the horizontal direction of the vehicle, and the cylinder block is joined with a block mounting surface formed at the outer peripheral surface of the crankcase. At the outer peripheral surface of the crankcase, the region forming a mounting block surface for connecting the cylinder block can be made broad without interfering with the transmission case. A degree of freedom can therefore be obtained for designing the angle of attachment of the cylinder block with respect to the crankcase and the power units.
In a second aspect of the present invention, journals supported by the crankcase are integrally formed with the crankshaft at just the two left and right ends thereof According to this second aspect, the crankshaft is integrally formed and the simplification of this structure is therefore achieved. Support of the crankshaft can therefore be carried out simultaneously with the joining of the first and second crankcase halves.
In addition to the first and second aspects, in a third aspect of the present invention, the transmission case is constructed by joining an inner transmission case half, integrally formed so as to extend from the first crankcase half, and an outer transmission case half positioned opposite the first crankcase half The partitioning surface is offset from a central position between the two cylinders to the side of the inner transmission case half. The width of the connected body of the first crankcase half and the inner transmission case half and the width of the second crankcase half are the same or substantially similar regardless of the inner transmission case being integrally formed with the first crankcase. Casting and cutting processes can therefore be carried out easily because the casting depth and processing depth is not deeper for one or the other of the connected body and the second crankcase.
In addition to the first to third aspects, in a fourth aspect of the present invention, an axis of the cylinder positioned on the side of the transmission case coincides with or is in the vicinity of a plane of the central rotation of a rear wheel. According to this fourth aspect, balancing of the weight to the left and right of the power unit can be achieved with respect to the rear wheel. This contributes to improved handling.
In addition to any one of the first to fourth aspects, in a fifth aspect of the present invention, the cylinders are arranged in a substantially flat manner with each head thereof facing towards the front of the vehicle. A low center of gravity can be achieved for the power unit. Further, as a broad space can be ensured above the engine, the power unit is appropriate for use in motor scooters and other two-wheeled vehicles with an accessory compartment arranged above the engine.
In addition to the first to fifth aspects, in a sixth aspect of the present invention, a timing transmission chamber is formed at the outer side of the cylinder and positioned on the opposite side of the transmission case. The timing transmission chamber houses a timing transmission assembly coupling the crankshaft and a moving cam shaft which is provided at the heads of the cylinders and being housed there. The spacing of the journals at both ends of the crankshaft can be kept to a minimum and the amount of overhang of an output end of the crankshaft linking to a transmission assembly can also be kept to a minimum.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.